Chlorella is a microalgae belongs to the phylum Chlorophyta. It contains high proteins, lipids, especially carotenoids, antioxidant and minerals. It has been produced commercially in several countries as a health food and dietary supplement. Chaetoceros is used as feed for larvae culture. These algae were culture by using media, in which sodium nitrate (NaNO3) used as nitrogen sources. Therefore, it is nessessary to find out the other nutrients in order to increase population growth of Chaetoceros. Two experiments were performed to replace NaNO3 by (NH4)2SO4 in Walne medium to culture Chaetoceros gracilis and Chlorella sp.. Culture in one litter Erlenmeyer flask with the conditions: water salinity of 25 ‰; continuous light and aeration supply. There were 4 treatments: (1) 32 mg/L of nitrogen from NO3+; (2) 32 mg/L; (3) 48 mg/L and (4) 64 mg/L of nitrogen from NH4+. The initial density of algae was 2 x106 cells/mL. Results showed that the maximal densities of C. gracilis were not significant differences between 32 mg/L from NaNO3 and 32 mg/L (NH4)2SO4 treatment. In the second experiment, Chlorella sp. population grew in 32 mg/L NaNO3 treatment as high as that in 48 mg/L (NH4)2SO4 treatment.